1. Field of the Invention
The present invention relates to a surface acoustic wave device for use as a resonator, a band filter, or other suitable device. More particularly, the present invention relates to an end surface reflection type surface acoustic wave device having a configuration in which an insulating film is formed so as to cover an electrode film which defines an IDT.
2. Description of the Related Art
As piezoelectric substrates for use in surface acoustic wave devices, 36° and 39.5° to 46° rotated Y plate LiTaO3 substrates are widely used. In particular, for RF surface acoustic wave filters, Al or Al alloys are used as materials for forming electrodes such as IDTs (Interdigital Transducers), and the film thickness thereof is approximately 0.08 λ to 0.10 λ when the wavelength of the surface acoustic wave is denoted as λ.
In this type of surface acoustic wave device, in order to improve the temperature coefficient of frequency (TCF), various configurations in which, on a piezoelectric substrate, an insulating film formed of an SiO2 film is formed so as to cover the electrode (see, for example, Japanese Unexamined Patent Application Publication Nos. 2-37815, 8-265088, and 9-186542).
In WO96/4713 and Japanese Unexamined Patent Application Publication No. 1999-186866, a configuration in which the top surface of the insulating film made of SiO2 is planarized is disclosed.
However, in a known surface acoustic wave device in which an insulating film made of SiO2 is formed, a stopband of a sufficient magnitude cannot be obtained. For this reason, when a surface acoustic wave resonator is formed, a large ripple appears in the vicinity of the anti-resonance frequency, and the anti-resonance point disappears. Furthermore, in the surface acoustic wave filter, there is the problem that filter characteristics are not sufficient.